1. Field of the Invention
The present invention relates to a wireless communication apparatus and a communication method using the same, and more particularly, to a wireless communication apparatus and a communication method capable of reducing the error rate of a connection conversion by improving communication flow during a conversion into a connection mode. The present application is based on Korean Patent Application No. 2001-80518, filed Dec. 18, 2001, which is incorporated herein by reference.
2. Description of the Related Art
Blue tooth is a wireless data communication technique employed in a range of fields in electrical communication, networking, computing, and consumer goods. Blue tooth can replace several cables required for connecting devices separated by a short distance. For example, when Blue tooth is realized in a mobile phone and a laptop computer, the devices can communicate with each other without using a cable. A printer, a PDA (personal digital assistance), a desktop computer, a facsimile, a keyboard, a joystick, and virtually all digital devices can be used in the Blue tooth system.
Generally, the fastest data transmission speed of Blue tooth is 1 Mbps and the maximum range of data transmission of Blue tooth is 10 m. 1 Mbps is an available transmission speed using a frequency within a range of the ISM (industrial scientific medical) frequency band of 2.4 GHz, which a user can use without permission and with low cost. Moreover, the transmission range of 10 m is great enough for the user to transmit data between a device and a PC in an office.
Furthermore, as Blue tooth is designed to be operated in a radio frequency band having a lot of noise, the data can be stably transmitted in a wireless frequency having much noise by using a frequency hopping method of 1600 times per second. The frequency hopping method is also called an FHSS (frequency hopping spread spectrum) method. In the FHSS method, a given frequency band is divided into many hopping channels, and is allocated to different hopping channels according to an order decided in advance when a signal (medium frequency) firstly modulated at a sending terminal is converted into an RF frequency band 2.4 GHz. Since the channel to which the signal is allocated is changed at a high speed, multi-channel interference and narrow band impulsive noise can be reduced. A receiving terminal restores an original signal by connecting the received signals after being allocated to several hopping channels in the same order as that of the sending terminal. 79 hopping channels are used for IEEE 802.11, and each hopping channel is disposed at an interval of 1 MHz. When the signal is allocated by hopping several channels, there should be at least a 6 MHz interval to avoid mutual interference between two temporally consecutive hopping channels, and the speed for changing the hopping channel (hopping rate) is more than 2.5 times per second.
Moreover, the Blue tooth system supports a one-to-multiple connection as well as a one-to-one connection. As shown in FIG. 1, in the Blue tooth system, a plurality of piconets can be constructed and connected, respective piconets being divided by different frequency hopping orders. The term “piconet” refers to a Blue tooth unit formed by connecting one or more slaves to one master device. A piconet can have one master and a maximum of seven slaves. The master device decides the overall characteristics with respect to the channel in the piconet. Blue tooth device address (BD_ADDR) decides a frequency hopping sequence and a channel access code. In other words, a clock of the master decides a phase of the hopping sequence and sets up timing. Moreover, the master controls the traffic on the channels. Any digital device can be a master, and once the piconet is configured, a master and a slave can change their roles.
The master and the slave basically perform bi-directional communication by a time division duplex method having a hopping slot of 625 μs ( 1/1600 second). A plurality of piconets connected with each other is called a scatternet.
FIG. 2 is a view showing communication by TDD between the master and the slave. Referring to FIG. 2, the length of each of the channels allocated to the time slot is 625 μs. The number of the time slot is decided in accordance with the Blue tooth clock of the piconet master. In addition, the master and the slave can selectively transmit a packet by the time slot. In other words, the master transmits the packet to time slots marked as even numbers, and the slave transmits the packet to time slots marked as odd numbers. Furthermore, the packet transmitted by the master and the slave should be realized within five time slots. Packet means a unit of data transmitted on the piconet channel.
In a Blue tooth connection state, the master can operate the slave in a hold mode, a sniff mode, or a park mode. In the hold mode, the slave connected with the master goes into a sleep state, while keeping an AM_ADDR (active member address). Additionally, in the sniff mode, a listening period is lengthened between the master and the slave that is connected with the master, while keeping an AM_ADDR. Moreover, in the park mode, the slave connected with the master enters into the sleep state after releasing the AM_ADDR. And before the slave moves to the park mode, the slave is allocated either a PM_ADDR (parked member address) or an AR_ADDR (access request address).
AM_ADDR is expressed as a member address, and recognizes active members in the piconet. In other words, when more than two slaves are connected with one master in the piconet, the master allocates a temporary three bit address to be used when the slaves are activated to recognize each slave. Therefore, the packets exchanged between the master and the slaves all carry AM_ADDR. In other words, an AM_ADDR of the slave is used not only for the packet transmission from the master to the slave but also for the packet transmission from the slave to the master. When the slave is not connected with the master or the slave is in the park mode, the allocated AM_ADDR is withdrawn. When the slave is connected with the master again, a new AM_ADDR should be allocated. The piconet should be limited to one master and seven slaves because the address allocated by a master to activate slaves is defined as three bits in the Blue tooth standard. In other words, address “000” among maximum eight addresses is reserved for broadcasting from a master to the slaves, and rest of the addresses from “001” to “111” are available.
Inquiry and paging processes are used to connect a new slave with the master. The inquiry process allows the apparatuses in the Blue tooth system to ascertain the address and clock of each of the apparatuses. The paging process is periodically performed by the master, and wakes up the slave. The response of the slave with respect to the paging process by the master is shown in FIGS. 3 and 4.
FIG. 3 is a view showing an initial connection when the slave responses a first paging message of the master. FIG. 4 is a view showing an initial connection when the slave responses a second paging message of the master.
When the paging message transmitted by the master is successfully transmitted, the hopping frequency number of the master and the slave is synchronized. The master and the slave maintain the connection state and enter into a response routine to communicate information. An important aspect of the piconet connection state is that the master and the slave use the same channel access code and the same channel hopping sequence, and the clocks of the master and the slave are synchronized. The channel access code and the channel hopping sequence are obtained from BD_ADDR of the master, and the timing is established by the clock of the master. To temporarily synchronize the clock of the slave to the clock of the master, offset is added to the native clock of the slave. When the connection is started, a master parameter should be transmitted from the master to the slave.
Referring to FIGS. 3 and 4, frequency f(k) and f(k+1) are frequencies of a hopping sequence of a page decided by BD_ADDR of the slave. Frequencies f′(k) and f′(k+1) correspond to a page response frequency from the slave to the master. Frequency g(m) is included in the channel hopping sequence.
Table 1 shows an initial message communicated between the master and the salve.
AccessHoppingcode andStepMessageDirectionsequenceclock1slave IDmaster to slavepageslave2slave IDslave to masterpage responseslave3FHSmaster to slavepageslave4slave IDslave to masterpage responseslave5first packet mastermaster to slavechannelmaster6first packet slaveslave to masterchannelmaster
In step 1, the master is in the paging state, and the slave is in the paging scan state. When the slave enters into the paging scan state, the slave selects a scan frequency corresponding to the page hopping sequence of the master. In this step, it is assumed that the page message (device access code of slave) transmitted by the master is received by the slave.
When the device access code is recognized, the slave transmits the response message in step 2. The response message transmitted by the slave is composed only of the device access code of the slave. The slave transmits the response message at 625 μs from the start of the received page message (ID packet of slave), and the hopping frequency of the response message coincides with the hopping frequency of the received page message. While the initial message is being communicated, the slave uses the page response hopping sequence to return information to the master. After transmitting the response message, a receiver of the slave waits for a FHS (frequency hopping synchronization) packet by being activated at 312.5 μs after the response message is started (step 3). As shown in FIG. 4, the FHS packet might be able to be transmitted 312.5 μs after the page message is transmitted when the slave responds to the second paging message of the master. In other words, in this case, the interval of 625 μs like in RX/TX timing is not applied.
When the FHS packet is transmitted to the slave in the slave response state, the slave returns a response configured only with the device access code of the slave to the master to inform of receipt of the FHS packet by using the page response hopping sequence (step 4). The transmission of the response packet is based on the FHS packet. Moreover, the slave alters the access code and the clock to the channel of the master transmitted from the FHS packet. In other words, in step 5, the slave enters into the connection state, and from that time, the slave uses the clock and BD_ADDR of the master to decide the channel hopping sequence and the channel access code. Connection mode is started by a POLL packet transmitted by the master. The POLL packet and the NULL packet have the same structure. However, while a NULL packet does not need to be responded to, a POLL packet requires a response from a receiving terminal indicating whether the receiving terminal has data to send. In addition, a POLL packet does not affect a response controller with ARQ (automatic repeat request) and SEQN (sequential numbering scheme) or re-transmission control method. A representative usage of a POLL packet is to examine the existence of the slave by the master in the piconet. When there is a slave, it responds to the master.
In step 6, the slave provides responses corresponding to the type of packet. After a FHS packet is received, when a POLL packet is not received by the slave during a new allocated connection number or the response packet is not received by the master, the master and the slave return to a paging and a paging scan state.
As described so far, for a Blue tooth system to establish a connection state, a page and a page scanning process should be successful in a setting-up step so that the slave can receive the clock value of the master. However, when the master cannot receive the ID packet of the slave due to, for example, excessive distance, an obstacle, or a defective RF module, the master consecutively transmits a FHS packet for a predetermined number of times as shown in FIG. 5. Yet, the slave has already entered into the channel hopping sequence state after receiving the FHS packet and sending an ID packet. Thus, the slave awaits receipt of the POLL packet by the channel hopping sequence. Therefore, the connection of the master and the slave cannot be further processed. After a predetermined time is passed, the connection set-up process ends in failure. Accordingly, the master and the slave should start from step 1 again, thereby causing more consumption of electrical power and lowering the connection rate.